Snow realized this presented a natural experiment, which he wrote about in an 1855 paper: “No fewer than three hundred thousand people of both sexes, of every age and occupation, and of every rank and station, from gentle folks down to the very poor, were divided into two groups without their choice, and, in most cases, without their knowledge.” He is effectively describing randomization, a practice crucial to modern epidemiology, but not widely during Snow’s era. He was just clever enough to realize that he could look back at where cholera occurred the last time it came to London, in 1849, and then wait for the disease to strike again. Prior to the change in water supply, people contracted cholera at roughly similar rates no matter where they got their drinks. If fewer people who sourced their water from the Lambeth Company got sick the second time around, that would suggest it was something in the water that made them ill.
That’s exactly what happened.
Cholera - Outbreak - August - Snow - Data
Another cholera outbreak began in August 1854, and Snow immediately began collecting data. His results showed that folks getting their water from Southwark and Vauxhall, which sourced water from within the city, had a death rate of 315 per 10,000 households. Those with the cleaner Lambeth Company water died at a rate of just 37. The rest of London...